1. Field of the Invention
The present invention relates to a copolyester useful for bottles, films or sheets. More particularly, the present invention relates to a copolyester which is capable of presenting a molded product with good productivity in its molding and having excellent heat resistance and mechanical properties and which shows excellent productivity in its production, and molded products made thereof.
2. Discussion of Background
Polyethylene terephthalate (hereinafter referred to simply as "PET") is excellent in the mechanical strength, chemical stability, transparency and sanitation and is light in weight and inexpensive. Therefore, it is widely used as packaging material in the form of various sheets and containers. Its use as containers for soft drinks, fruit juices, liquid seasonings, edible oils, liquors and wines has been particularly remarkable in recent years.
For example, in a case of a bottle, such PET is molded into a preform for a hollow molded product by an injection molding machine, and this preform is then subjected to stretch blow molding in a mold having a predetermined shape. In a case where the content of the bottle requires hot filling as in the case of a fruit juice beverage, it is common to conduct heat setting in the blow mold or in a separate mold to form a heat set bottle.
However, conventional PET chips used for molding usually contain oligomers in an amount of from 1 to 2% by weight in the case of melt polymerization chips and from 0.5 to 1.0% by weight even in the case of solid-state polymerization chips, as the amount of a cyclic trimer as the main component of the oligomers. Such oligomers tend to deposit on and contaminate the apparatus such as the mold during the molding operation. Contamination of the mold, etc. is likely to cause blushing or surface roughening of the resulting molded product. Therefore, it is necessary to frequently clean the mold, etc.
Heretofore, it has been attempted to reduce oligomers by prolonging the solid-state polymerization time or by increasing the amount of the catalyst. However, reduction of oligomers by such a method is rather limited and is not economical.
On the other hand, many copolyesters having properties similar to PET, such as a copolyester using terephthalic acid and isophthalic acid as the dicarboxylic acid components and a copolyester using ethylene glycol and diethylene glycol as the diol components, are also known. However, no copolyester has been specifically known with the oligomer content being reduced to a certain extent and having physical properties equal or superior to PET. Further, it has not been known that by copolymerizing a small amount of isophthalic acid to PET, the oligomer content of the copolyester obtainable by melt polymerization, followed by solid-state polymerization, can be lowered as compared with homopolymerized PET.